Rivers Flashcards
What type of erosion happens more in the lower course?
- lateral erosion
- gradient is flatter so river meanders find the best route to sea
- so more erosion on outer part of river
What is a meander?
a bend in the river’s channel
What are the main features of a meander?
- thalweg
- deposition on the inside bend
- erosion on the outside bend
- river beach/slip-off slope
- river cliff
How are meanders formed?
- river erodes laterally & migrates across valley floor over time, widening valley
- when river reaches meander, most water goes to outer bend & erosion occurs here, since there is less friction as the water is deeper
- abrasion & hydraulic action undercut outside of river bend forming a river cliff
- sand & small pebbles are deposited creating a gentle slip off slope on the inside bend since water is shallower & there is more friction
- this continues
- over time, meander will become tighter forming a swans neck & possibly oxbow lake
How is an oxbow lake formed?
- continued erosion on outer bend of meander where river flows fastest
- deposition continues on inside bend, causing slip off slope to grow
- so outside bends of meander move closer together as meander neck becomes narrower
- swan’s neck meander as 2 meanders move closer together
- during a period of high discharge (maybe a flood) river cuts through neck to shorten the source
- fastest current now flows in centre of channel & deposition means original meander is blocked off leaving an oxbow lake
Describe the characteristics of a river’s drainage basin?
- rivers begin upland & flow downstream, become wider until the enter the sea
- source is where the river begins & where it ends is the mouth
- other smaller rivers (tributaries) join the main river at the confluence
- the area drained by a river is the drainage basin
- the boundary of this is the water shed, often a ridge of high land
What is the hydrological cycle?
- evaporation
- transpiration
- condensation
- precipitation
- surface run-off
- infiltration
- percolation
- groundwater flow
- throughflow
What is precipitation?
any source of moisture reaching the ground, e.g. rain, snow, hail
What is transpiration?
water loss through pores in vegetation
What is throughflow?
water flowing through the soil layer parallel to the surface
What is groundwater?
water stored in the rock
What is soil moisture?
water held in the soil layer
What is surface storage?
water held on the ground surface e.g. puddles
What is percolation?
water seeping deeper below the surface
What is groundwater flow?
water flowing through the rock layer
What is evaporation?
water lost from the ground surface
What is surface run off?
water flowing on top of the ground
What is infiltration?
water sinking into soil/rock from the surface
What is interception?
water being prevented from reaching the surface by trees & grass
What is a floodplain?
flat area adjacent to the river channel, mainly in the lower part of the course
What are levees?
raised banks along the course of the river in the lower course, formed naturally but can be artificially increased in height
How are floodplains formed?
- river widens by lateral erosion, taking away interlocking spurs present nearer the source & a flat, wide area is created
- at times of high discharge, river has high amounts of energy, which transports large amounts of material
- when river overflows it will spread out across the surrounding flat land
- sudden increase in friction will reduce the water’s velocity & fine silt will be deposited
How are levees formed?
- each time the river floods, another layer of silt is added, forming a flat floodplain
- largest material will be deposited first, forming a natural embankment (a levee)
What are deltas?
found at mouth of river, where it meets the sea, usually triangular in shape with top of delta having flat surface
How are deltas formed?
- sediment brought downstream, as velocity decreases deposition occurs
- heaviest load deposited first
- finer particles carried into deeper water & when dropped divides river into separate channels called distributaries
- because there is no current the material grows to sea & the distributaries rejoin
- deposition continues so it builds upwards & outwards, over time vegetation will colonise the area
How are interlocking spurs formed?
- vertical erosion in upper course creates v-shaped valley, that is steep sided & narrow
- as river erodes downwards, soil & loose rock on valley sides are moved downhill by slopewash/soil creep
- material is then added to load of river
- river has to wind around hills, called interlocking spurs
How are waterfalls formed?
- layer of hard rock lays over a layer of soft rock, water begins to erode soft rock that is less resistant, creating a steep gradient
- erosion of less resistant rock underneath, undercuts hard rock above, hydraulic action & abrasion create a plunge pool at foot of waterfall
- ledge that overhangs plunge pool eventually collapses under it’s own weight, debris is then eroded by hydraulic action & attrition, material is then transported downstream
- waterfall takes a position further back
What is a gorge?
very steep sided valley
How is a gorge formed?
as a waterfall continues to retreat upstream
What are potholes?
if a river bed is uneven, pebbles can become trapped & swirling currents cause the pebbles to rotate & erode circular holes in the river bed
What is the channel?
part of river valley occupied by water
What is hydraulic action?
when air is forced into cracks in river banks by flow of water, causing them to widen & break down
What is attrition?
load carried by the river collides causing it to break into smaller, more rounded pieces
What is abrasion/corrasion?
load carried by river rubs against river bed & banks wearing them away
What is solution/corrosion?
some rock minerals slowly dissolve in river water, wearing river bed & banks away
What is load?
material carried by river
What are the 2 directions of erosion?
- vertical (down) - upper course
- lateral (sideways) - lower course
What is the wetted perimeter?
amount of water that comes into contact with the rivers banks & bed
How do you calculate wetted perimeter?
add up length & width of channel
What is traction?
large boulders & rocks rolled along river bed
What is saltation?
small pebbles & stones bounced along river bed
What is suspension?
fine, light material carried along by river
What is solution?
minerals dissolved in water, this is a chemical change
When does deposition occur?
when river doesn’t have enough energy to carry its load, when river slows down & gets shallower
Where is deposition most likely to occur?
at mouth, inside bend of meander, flood plain
What factors affect river discharge?
- temp.
- vegetation levels
- urban areas
- rainfall
- rock type
- slope
How does temperature affect river discharge?
higher temp. causes snow to melt, increasing river discharge
How do vegetation levels affect river discharge?
high amounts can cause interception, decreasing discharge & increasing lag time
How do urban areas affect river discharge?
contain impermeable surfaces & can cause an increase in river discharge due to lack of infiltration & increased surface run off, decreasing lag time
How does rainfall affect river discharge?
high levels can increase discharge, causing a decrease in lag time due to ground being saturated
How does rock type affect river discharge?
impermeable rock types cause increase in river discharge as water flows over land to river, causing a decrease in lag time
How does a slope affect river discharge?
steep slopes can increase discharge & decrease lag time as water runs to river faster
Which river management strategies are soft management?
warning systems
flood plane zoning
afforestation
river restoration
dredging
Which river management strategies are hard management?
dams
artificial levees & embankments
straightening
flood relief channels
What are warning sytems?
- websites, weather apps
- FEWS - flood early warning systems
- plan evacuation route
- add sandbags
-reduce loss of life - homes will still be flooded
What are flood plane zones?
- don’t build properties right next to river
- prevent damage to property
- some might not be able to access zoning
What is afforesation?
- plant trees to increase lag time - interception decreases discharge
- low cost
- not that effective
- long time - trees grow slowly
- counter deforestation
- restores natural habitat
What is river restoration?
- low cost
- makes quality worse
- restores river to its natural state
- increases erosion
- expensive initial cost
What are dams?
- barrier that stops/restricts flow of water
- used for irrigation, tourism (boating on reservoir), hydroelectric power
- expensive
- disrupts fish habitat/migration
What are artificial levees/embankments?
- increase banks of river - reduce discharge
- prevent lateral erosion
- expensive
- ruin look
What is straightening?
- removal of meanders - can hold more volume
- high volume so water can pass through quicker
- increases risk downstream
What is dredging?
removal of silt
What are flood channels?
- artificial channels to divert water (decreases discharge)
- pressure of main channels
- hard to find land
- expensive
- if empty can be a place of litter
What does the valley cross section of an upper course of a river look like?
- vertical erosion
- steep sided valley
What does the channel cross section of an upper course of a river look like?
- low energy so can’t erode/transport boulders
- rough channel bed - friction
What does the valley cross section of a middle course of a river look like?
- lateral erosion
- channel occupies far less of total valley
What does the channel cross section of a middle course of a river look like?
- smoother channel bed
What does the valley cross section of a lower course of a river look like?
- levees/raised banks
- wide, flat valley
What does the channel cross section of a lower course of a river look like?
- very smooth channel: low friction, high velocity
How does a river’s cross profile change from source to moutg?
- upper course: valley & channel are narrow & deep due to large amount of vertical erosion
- sides of river’s valley are steep (v-shaped valley)
- channel occupies majority of valley floor
- more downstream: valley increases in width
- channel depth doesn’t change much as vertical erosion as slowed
- sides of valley become more gentle
- rivers channel occupies little of valley floor
What opportunities are associated with rivers
- agriculture - fertile silt & soil, so areas are intensively farmed, repeated flooding replenishes nutrients taken by plants, provides water for irrigating crops
- trade - many successful cities built by rivers to provide trade routes to other countries & continents
- leisure - boating, rowing swimming…
- energy - source of power for mills & factory machinery, modern tech allows electricity production from large dams e.g. 3 Gorges dam (China)
- homes - floodplains provide flat building land, river water can be used for drinking, washing & cooking
What do long profiles show?
how a rivers gradient changes
What do cross profiles show?
a cross-section of a channel & valley at certain points in rivers course
What would a long & cross profile show of the upper course of a river?
- steep gradient
- low total load
- slow average velocity
- vertical erosion
- big particle size
- rapids/waterfalls
What would a long & cross profile show of the middle course of a river?
- less steep
- increasing total load
- increasing average velocity
- increasing discharge/volume
- decreasing particle size
- meanders/oxbow lakes
- lateral erosion
What would a long & cross profile show of the lower course of a river?
- flattest gradient
- highest total load
- smallest particle size
- fastest average velocity (until mouth)
- flood planes & deltas
Why do we care about flooding?
- can cause damage to homes/businesses, infrastructure & communications
- social problems - homelessness/injury/death
- economic problems - cost of repair/loss of businesses
- environmental problems - damage to crops/destroying animal habitats
What are hard engineering strategies?
building artificial structures that try to control rivers
What are soft engineering strategies?
doesn’t involve building artificial structures, but takes a more sustainable & natural approach to managing potential for flooding
What is lag time?
shorter the lag time, flashier the flood
What is alluvium?
fine, fertile sediment depostied on a flood plane
What is the Mekong Delta used for?
- domestic use (cooking & cleaning)
- transport
- irrigation (moving water to fields to water crops)
- trade & tourism (floating market)
Statistics about the Mekong Delta
- one of the most intently farmed areas in Asia
- Vietnam is 2nd biggest rice exports in the world
- 80% who live there are farmers
- catching 50kg fish per day = $70,000 per year
- GDP per capita >$4,000
- 90% Vietnam’s population live in countryside using waters of the Mekong
- 11 dams in lower course of Mekong provides electricity for millions in Vietnam & Laos
Where are the Somerset Levels?
coastal plains & wetlands in Somerset, SW England
What rivers flow through the Somerset Levels?
River Tone & Parrett, drain into Bristol channel
What were the physical causes of the Somerset Floods?
- rain - low pressure caused heavy rain off the Atlantic, causing 350mm rain In Jan & Feb (10mm above average)
- high tides - storm surges caused high tides, preventing freshwater from draining into sea
- dredging - hadn’t been dredged in over 20 years
What were the human causes of the Somerset Floods?
- much land been converted from grassland to grow maise
- ground able to retain less water
- causing surface runoff
What were the social impacts of the Somerset Floods?
- over 600 homes flooded
- 16 farms evacuated
- power supplies cut off
- villages (e.g. Moorland) cut off completely & people left stranded
What were the social impacts of the Somerset Floods?
- cost damage estimated over £10m
- over 140km² agricultural land underwater for 3-4 weeks
- 1000 livestock evacuated
- Bristol to Taunton railways line closed
What were the environmental impacts of the Somerset Floods?
- sewage, chemicals (e.g. pesticides) contaminated floodwater & it spread to other areas
- destroyed ecosystems & limited food supply for animals
What were the immediate responses of the Somerset Floods?
- warning (amber alert from MetOffice)
- placed sandbags in front of buildings
- man in Moorland made large wall out of clay & mud to protect his house from flooding
- 80 homes evacuated
- fire brigade rescued people on boats
- 65 pumps used to drain 65mil m³ floodwater
- government provided £15m
What were the long-term responses of the Somerset Floods?
- Somerset Levels & Moors Action Plan reintroduced dredging, constructing a tidal barrage & additional permanent pumping stations
- part of a 20 year plan for Somerset Levels that will total £100m
